In this work, the reduction of the moisture content from waste oils by a bentonite-based adsorption process was investigated as an alternative for reuse in energy production. Waste cooking oil (WCO) and waste poultry fat (WPF) were characterised (moisture and viscosity, values of acidity, saponification, and peroxide), as well as bentonite clay adsorbent. The response surface methodology (RSM) was applied to define the best moisture adsorption condition by bentonite-based adsorbent regarding a full factorial experimental design (FFED). In the framework of RSM, the adsorbent mass, oil acidity, and agitation speed were varied in three levels in the range of 1.5–3.0 g, 1–38 mgKOH goil−1, and 50–150 rpm, respectively, while the adsorption time was fixed in 24 h. Under the best moisture adsorption condition, kinetic tests were performed in a 1–24 h contact time range, besides performing equilibrium adsorption tests at 5 h contact time. Around 90% moisture removal was attained with 1.5 g adsorbent, acidity value of 1 mgKOH goil−1, and 50 rpm agitation speed, achieving the equilibrium in 5 h. A multilayer adsorption process, as described by the BET isotherm model, was acting in dried bentonite clay, allowing getting excellent moisture adsorption capacity (∼200 mgwater gBent−1). Therefore, moisture removal from waste oils by a bentonite clay-based adsorption process has provided results that are suitable and recommendable for economically viable biodiesel production.